Conventional physiological monitoring relies on the so-called vital signs of heart rate, blood pressure, core temperature, respiration rate, and more recently oxygen saturation. Due to their long history and widespread familiarity, conventional vital signs are the parameters of choice in physiological monitoring. The parameters evaluated through the vital signs are highly regulated by the organism in order to maintain life. While average values for the vital signs are constrained within a relatively narrow range during normal life, vital sign dynamics can provide insight into regulatory failure or regulatory compensatory actions as underlying physiological challenges are addressed by the organism.
There is growing interest in monitoring additional parameters in an attempt to better define physiological status using easily measured indications. One such group of parameters is variability, e.g., arterial blood pressure variability, heart rate variability, QT interval variability, etc. Such parameters can be extracted from high precision vital sign measurements. Current indications are that variability measures may provide a sensitive means of identifying physiological control indicators of challenges to homeostasis. Unfortunately, the interpretation of variability is complex due to challenges in effective parameterization.